Sophisticated methods based on surface phenomena have been recently developed as mechanisms to recover additional crude organic fuel reserves that could not be recovered through primary production mechanisms. Such processes cannot be duly evaluated if the effects exerted by temperature and pressure on the interfacial tension of hydrocarbons present in a porous medium are not known.
The measurement of interfacial tension at reservoir conditions is useful to determine the interaction that develops at the gas-crude interface and to evaluate how interfacial forces are altered when a chemical additive is added to any phase. Nevertheless, scientists and engineers have been faced for years with the challenge of obtaining a precise measurement of interfacial tension. Among the existing methods to measure the interfacial tension of an equilibrium system the pendant drop method has recently become popular, particularly where it is necessary to measure very low values thereof. This method is based on the formation of a liquid drop on the end of a capillary; the drop outline and the capillary are photographed and the interfacial tension is calculated on the basis of their size and the fluid density under the established experimental conditions.
The apparatus used to obtain the above described data includes a visual cell capable of supporting high pressures and temperatures, and a capillary to form the drop within the cell. In most of the systems previously developed, when a different size drop was to be generated and a different capillary was required, it was necessary to break down the equilibrium conditions within the apparatus by lowering the pressure and temperature in order to uncover the cell and exchange the capillaries. In those operations where relatively high pressures of the order of about 5,000 psia at a temperature of about 300 F were involved, the continual exchange of capillaries represented a serious problem.
In U.S. Pat. No. 3,483,737 to Jennings, Jr. et al., a visual cell is described which includes several capillaries that can be selected to form the drop without varying the pressure and temperature conditions. The capillary selection system in the cell described by Jennings, Jr. et al. is based on internal ring and pinion gears and, preferably, opposed pairs of such gears. That gear arrangement and operation was cumbersome and could pose condensate retention problems which are difficult to control. This, in turn, can considerably affect the gas-liquid interfacial tension measurements for such tension is quite sensitive to impurities. Additionally, there is no provision for precise control of the drop's size, since the drop is controlled through piston-type action exerted by the sample fluid pressure source. Further detailed background information on the equipment and methods heretofore most frequently used to determine interfacial tension can be found in Andreas et al., "The Journal of Physical Chemistry" 42:1001(1938); and Jennings, Jr., "The Review of Scientific Instruments" 28:744-777(1957).